In areas of the country subjected to freezing wintertime temperatures it is necessary to prepare inground swimming pools for such temperatures in order to avoid extensive damage, particularly to those pipes and conduits that are buried in the ground but above the frost line. Water left in those pipes and conduits will freeze and expand, damaging the pipes and fittings. Normally the winterizing process involves the steps of draining water from all pipes situated above the frost line, closing exposed openings in the pool side wall to prevent ingress of water, debris, rain or snow, and removing or protecting outside mechanical equipment.
In most installations the water inlets into the pool are situated well below the normal water line, perhaps by as much as 30 to 50 cm. With standard winterization procedures, often taking from 6 to 8 hours depending on the pool size, one must first of all reduce the water level to below the inlets so that the water remaining in the pipes above the frost line can be drained therefrom without fear of replacement water entering via the inlets. Draining to that level can involve the removal of several thousand liters of water from a typical residential swimming pool, perhaps as much as 25,000 liters. In addition to being a wastage of water, the drained water usually is fed into a neighbourhood storm sewer system. When many pool owners are winterizing at the same time the sewer system can be severely loaded by such pool drainage water. Also, storm sewer water is not normally treated and the chemicals used in swimming pool water can be harmful to the environment. Additionally, the drained water must be replaced after winterizing is complete to protect the pool structure from winter damage. It is therefore desirable to reduce, as much as possible, the volume of water that must be drained from pool during the winterization process.